Wireless cellular telecommunication networks are known. The area covered by the network is divided into a plurality of cells. Each cell is served by a base station which is arranged to receive signals and transmit signals to mobile stations located in the cell associated with the respective base station.
In mobile stations, the receiver is arranged to perform synchronisation with respect to the received signals. This synchronisation can be divided into two stages. Initially the signal is acquired during the acquisition phase and the initial synchronisation is performed. In the next stage, the signal is tracked. In particular, changes in the radio channel and the receiver are tracked so that synchronisation is maintained.
The following is an example of frequency synchronization. For timing, a similar arrangement can be used.
In the known receivers, this synchronisation is done for example for frequency and timing. Reference is made to FIG. 2 which shows a block diagram of a known receiver. The signals are initially received by an antenna 2. The output of the antenna 2 is input to a first bandpass filter 4 which filters out the unwanted signals. Typically, the first bandpass filter 4 will allow a relatively wide range of frequencies therethrough. The output of the first bandpass filter 4 is output to a mixer 6. This mixer 6 downconverts the received radio frequency signals to a baseband frequency. This is achieved by mixing the received signal with an appropriate mixing frequency. This will be described in more detail later.
The output of the mixer 6 is input to a second bandpass filter 8 which is much narrower than the first bandpass filter. This second filter 8 is arranged to remove unwanted signals falling outside the bandwidth of the second filter. The output of the second filter 8 is input to an analogue to digital converter 10 which converts the signals from analogue to digital form. The output of the converter is output to a digital processor 12. The digital processor 12 has a detector 14 which estimates the frequency and generates a correction factor.
The correction factor is output to a third filter 16 which filters the correction value. It should be appreciated that the correction value is a digital value. The filtered corrected value is output by the digital processor 12 to a digital to analogue converter 18. The converter 12 converts the digital correction value to an analogue value. This analogue value is used to control the mixer 6 and in particular the frequency with which the output of the first bandpass filter 4 is mixed. This controls the frequency of the signals which are output by the mixer 6.
As can be seen, the synchronising elements include digital elements and analogue elements and as such are sometimes referred to as hybrid synchronisers. This can lead to problems. In particular the correction value is determined in the digital domain but the correction is done in the analogue domain. The conversion of the correction value from the digital value to the analogue value has problems associated therewith. In particular, the digital to analogue converter is not linear so the angular coefficient is not constant. Additionally, the analogue correction will suffer inaccuracies due to temperature changes, aging and operating conditions. For example the correction will vary depending on the frequency. Thus the correction made by the analogue elements will not be particularly accurate. This may mean that the variance caused by the correction is greater than the variance in the parameter estimation done in the digital processor.
A further problem is caused by the factor that the digital value is converted into an analogue value. The number of bits of the correction value provides a limitation on the accuracy of the correction. If the digital to analogue converter is able to deal with a relatively long word, it will be relatively expensive. If on the other hand the digital to analogue converter is able only to deal with relatively short words, the correction can only be done by limited step sizes. This problem is referred to as quantization noise.
Reference is made to FIG. 3 which shows a graph of the control value as a function the control step in a non-linear system. If this function shown in FIG. 3 is well known and the minimum step is small enough, it is possible to calculate the control word so that precise control can be achieved. in practice, this is difficult to do in that this function is dependent on temperature, ageing and operating conditions. This means that even if the receiver is tuned in the factory to achieve optimum performance, this optimum performance will not be achieved once the receiver is actually used. Additionally, as mentioned previously, the minimum step size is preferred to be relatively large to minimise the costs of the digital to analogue converter.
It has been proposed to provide two loops to provide control. One loop incorporates a digital automatic frequency control which provides a fine correction. The other loop provides a rough correction. The two loops are independently controlled. The first loop is faster than the second loop. However this arrangement also has problems. The first loop can not react fast enough and hence there is a transient when second loop is controlled. For example if the rough control loop has a 500 Hz step and the fine control loop has a 100 Hz control loop problems arise. The first loop is unable to provide a reading if the error is greater than 100 Hz. Additionally the problem of the unknown step size means that the digital correction is not able to work correctly when the analogue control word changes. The problem is that while the frequency is estimated in the decision directed loop which requires reliable decisions, only rather small quantization steps are allowed. Otherwise the frequency error would be so large that the frequency estimation could fail in especially poor conditions and the synchroniser could become unstable.
An additional problem of acquisition is that this should be done quickly. This usually means that from time to time large corrections have to be made. This causes additional problems to those which have already been discussed.
In summary, the large unknown time varying step size in the analogue correction causes a number of difficulties, as discussed above.